This invention relates to a charge plate for use in a laminated coating head of the general type described in Beam et al U.S. Pat. No. 3,586,907. Coating heads of that type are used in ink jet printing systems, which create printed matter by selective charging, deflecting and catching of drops produced by one or more rows of continuously flowing ink jets. The jets themselves are produced by forcing ink under pressure through a series of orifices in an orifice plate, which is one component of the laminated head.
A stimulation arrangement stimulates the jets to break the ink up into uniformly sized and regularly spaced drops, with drop formation occuring in all jets at more or less fixed positions, all located approximately the same distance from the orifice plate. The charge plate is positioned within the coating head so as to achieve electrical charging of selected ones of the drops being generated.
A charge plate as taught in the Beam et al patent comprises a plate of dielectric material provided with a series of charging tunnels located equidistantly along a straight line. Each charging tunnel is coated with an electrically conductive material so as to define a cylindrical charging electrode. Electrical leads are connected to each such charge electrode, and the electrical leads in turn are activated selectively by an appropriate data processing system. Typical prior art charge plates including such electrodes are disclosed in Solyst U.S. Pat. No. 3,975,741, in Kuhn U.S. Pat. No. 3,984,843 and in Bassous et al U.S. Pat. No. 4,047,184. The prior art also includes charge plates having charging electrodes formed in notches along the edges of the plate, as disclosed in the above mentioned Solyst patent, and also in Robertson U.S. Pat. No. 3,604,980, Culp U.S. Pat. No. 3,618,858, and in Van Breemen et al U.S. Pat. No. 4,035,812.
A serious difficulty arises in the fabrication of a suitable charge plate because of a requirement that the charging tunnels be accurately positioned, located extremely close together, and have a very high length to diameter ratio. For a typical ink jet printer as used in the forms printing business, the charging electrodes may be arranged in two rows, with the electrodes in each row having a center-to-center spacing of about 0.423 mm. In order to accommodate jets the internal diameter of the charging electrodes may be about 0.355 mm. diameter, so that the bridge between electrodes is only 0.068 mm. thick. Moreover, in order to accommodate variations in the filament lengths of the jets, the charge plate must be at least about 1 mm. thick. This means that each of the tunnels has a length to diameter ratio in excess of 2.8. Depending upon the width of the area to be printed, there may be anywhere from several hundred to over one thousand such cylindrical electrodes, each of which must be located with highest accuracy in relation to a reference point on the plate.
Due to the close spacing of the tunnels and the high length to diameter ratio it has been extremely difficult to fabricate suitable charge plates. Drilling of the tunnels has been found to be extremely expensive and largely unsatisfactory in plate materials having sufficient rigidity for the present application. Casting has not been satisfactory, due to the poor dimensional stability of known casting materials suitable for this application.
The most satisfactory charge plate thus far used for such purposes has been photofabricated from a photosensitive ceramic material, which is exposed, etched, and thereafter fired to a final state. It has been found that the firing process causes dimensional alteration of the plate, so that most of the plates have to be rejected. Those plates which do pass inspection are in many cases marginally acceptable, and they are in any event quite delicate and easily damaged. Moreover it has been difficult to plate charge tunnels and electrical circuitry on such plates in a fully satisfactory manner.